Lyocell Method Comprising an Adjustment of the Processing Duration Based on the Degree of Polymerization

ABSTRACT

The invention relates to a method and a device for producing Lyocell fibres which are extruded from a cellulose solution of water, cellulose and tertiary amine oxide in a spinning head ( 25 ). The cellulose solution is obtained in a number of process steps directly from the cellulose ( 3, 4 ) or with the formation of a cellulose suspension. Since the degree of polymerisation decreases in the course of processing the cellulose through to its extrusion in the cellulose solution, with the processing of celluloses with a low degree of polymerisation there is the risk that the endless moulded bodies ( 2 ) extruded in the spinning head ( 25 ) exhibit defective quality. In order to also be able to process celluloses ( 3, 4 ) with a low degree of polymerisation without having to accept reduced quality, according to the invention, the residence time of the cellulose from its introduction into the device ( 1 ) through to the extrusion of the cellulose ( 3, 4 ) in the cellulose solution is set in dependence of the degree of polymerisation of the cellulose, of the cellulose suspension and/or of the cellulose solution.

The invention relates to a method of producing Lyocell fibres, in whicha cellulose is introduced with a predetermined degree of polymerisation,and from the cellulose, with the addition of a treatment medium, acellulose solution or initially a cellulose suspension and from thisthen the cellulose solution is produced, and in which the cellulosesolution is extruded to form endless moulded bodies.

The invention also relates to a device for the production of Lyocellfibres with a mixing device to which a cellulose can be fed and in whicha cellulose solution, with the addition of a treatment medium, directlyor with the formation of a cellulose suspension can be processed, with aspinning head, through which the cellulose solution can be extruded toform endless moulded bodies, and with a conveying device, through whichthe cellulose suspension and/or the cellulose solution can be conveyedfrom the mixing device to the spinning head.

These types of methods and devices are known from the Lyocelltechnology. With the Lyocell technology threads, fibres, films andmembranes are extruded as endless moulded bodies from the spinning masscontaining cellulose, water and tertiary amine oxide. Due to itsenvironmental compatibility, the Lyocell technology is increasinglyreplacing the conventional viscose methods. The environmentalcompatibility of the Lyocell method is based on the solution of thecellulose without derivatisation in an organic, aqueous solvent. Fromthis cellulose solution endless moulded bodies are extruded, for examplefibres and films. Through the extrusion of the moulded bodies and theorientation and regeneration of the cellulose in the course of theextrusion moulded bodies of high strength are obtained with versatilepossible uses in the textile and non-textile sector. The name Lyocellwas issued by the BISFA (International Bureau for the Standardisation ofMan-made Fibres). In the state of the art, the Lyocell method is nowwell documented.

Tertiary amine oxides are known as solvents for cellulose from U.S. Pat.No. 2,179,181 which can dissolve cellulose without derivatisation. Fromthese solutions, the cellulose moulded bodies can be obtained byprecipitation.

The processing of the cellulose, dissolved in an aqueous amine oxide,particularly N-methylmorpholine-N-oxide (NMMNO), is howeverproblematical with regard to safety, because the degree ofpolymerisation of the cellulose on dissolving the cellulose in NMMNOdecreases. In addition, amine oxides generally exhibit only limitedthermal stability, particularly in the system NMMNO/cellulose/water, andhave a tendency to spontaneous exothermic reaction. To overcome theseproblems and to be able to manufacture Lyocell fibres economically,there is a series of methods for solution in the state of the art.

In U.S. Pat. No. 4,144,080, it is stated that at high temperatures thecellulose dissolves more quickly in a tertiary amine-N-oxide and forms amore homogeneous solution if the cellulose together with the preferredingredients of tertiary amine-N-oxide and water is milled. InWO-94/28219, a method for the production of a cellulose solution isdescribed in which milled cellulose and an amine oxide solution areplaced in a horizontal, cylindrical mixing chamber. The mixing chamberexhibits axially spaced stirring elements that are rotating around itslongitudinal axis. Apart from NMMNO, N-methylpiperidine-N-oxide,N-methylpyrolidone oxide, dimethylcyclohexylamine oxide and others canbe used as the amine oxide. Mixing in the mixing chamber occurs between65° C. and 85° C. According to WO-A-98/005702, the cellulose is mixed ina device with the aqueous solution of the tertiary amine oxide, wherebythe mixing device exhibits a mixing tool and a container which rotatesduring mixing.

In WO-A-98/005702, the mixing tool is improved such that it is formed asa paddle, rail or helix and during mixing preferably prevents theformation of deposits on the inner surface of the container. InWO-A-96/33934, a buffer device is described, which comprises a mixingvessel and a conveying screw as a discharging device. In this way, acontinuous production of the cellulose solution is facilitated despitethe cellulose being fed in batches.

The method of WO-A-96/33934 has been further developed by the method ofWO-96/33221, in which a homogeneous cellulose suspension is producedfrom pulverised cellulose and an aqueous amine oxide solution in onesingle step. For this purpose, the pulverised cellulose is brought intocontact with the liquid, aqueous tertiary amine oxide and a firstmixture is formed in this way. The first mixture is spread in layers ona surface and transported under intensive mixing over this surface. Thisprocess can be carried out continuously. Other methods in which thecellulose solution is treated in the form of a thin layer are also knownfrom EP-A-0356419, DE-A-2011493 and WO-A-94/06530.

Also the pulverisation of the cellulose itself is an object of patentpublications. For example, U.S. Pat. No. 4,416,698 mentions it as anadvantage if the cellulose is milled to a particle size of less than 0.5mm. In WO-A-95/11261, prepulverised cellulose is introduced into anaqueous solution of a tertiary amine oxide to produce a firstsuspension. This first suspension is then milled and converted into aformable cellulose solution with the application of heat and a reducedpressure. In order to feed back the dust arising from milling orpulverising the cellulose into the process, filters are used inWO-A-94/28215 through which the cellulose dust is separated from theair. In WO-A-96/38625 a system is described which can pulverise bothcellulose bales as well as cellulose in leaf form. For this purpose, anejection hopper is provided which opens into a device for prepulverisingthe cellulose.

In EP-B-0818469, it is suggested that cellulose is dispersed in aqueousamine oxide solutions and the dispersion thus obtained treated withxylanases.

Apart from these efforts to economically produce a homogeneous cellulosesolution capable of being spun, there are also attempts to overcome theproblem of the decomposition phenomena of the cellulose solution whichoccur spontaneously under an exothermic reaction. In Buijtenhuis et al.,The Degradation and Stabilisation of Cellulose Dissolved in NMMNO, in:Papier 40 (1986) 12, 615-618, investigation results are described,according to which metals appear to reduce the decompositiontemperatures of the NMMNO in the cellulose solution. Primarily, iron andcopper appear to speed up the decomposition of NMMNO. Other metals, suchas for example nickel or chrome, also exert a negative influence on thedecomposition properties of the cellulose solution in appropriateoccurrence and appropriate concentration, if they are present inappropriate concentrations. However, in WO-A-94/28210 stainless steel isstill used as the material for a spinning head in order to withstand thehigh pressures during the extrusion of the cellulose solution.

In addition, the system NMMNO/cellulose/water in the highly concentratedNMMNO region has the property of releasing metal ions from the processapparatus, such as lines, filters, and pumps, which reduces the systemstability. In WO-A-96/27035, a method for the production of cellulosemoulded bodies is described in which at least some of the materials incontact with the cellulose solution contain at least 90% of an elementfrom the group of titanium, zirconium, chrome and nickel down to a depthof at least 0.5 μm. The important aspect with regard to WO-A-96/27035 isthat the rest of the composition of the apparatus and piping, where itcomes into contact with the cellulose solution, does not contain anycopper, molybdenum, tungsten or cobalt. According to WO-A-96/27035, thismeasure should prevent exothermic decomposition reactions.

Finally, in DE-C-198 37 210, which is taken as the closest state of theart, a homogeneous cellulose solution is produced irrespective of thewater content of the cellulose used. In contrast to current methods,here the cellulose is first transported through an initial shear zone inthe absence of NMMNO under homogenisation in a pulper and is only thenadded to a low water-content NMMNO.

Another way of producing the cellulose solution is followed in DE-A-4439 149 which forms the closest state of the art. According to the methodof DE-A-44 39 149, the cellulose is pretreated enzymatically. Toincrease the effectiveness of the enzymatic pretreatment, the cellulosecan be disintegrated before the pretreatment under shearing in water.Then, the pretreated cellulose is separated from the liquor and theseparated cellulose is introduced into a melt of NMMNO and water.Hereby, the separated liquor can practicably be fed back to thepretreatment after supplementing the water and enzyme losses. However,in practice, this type of process management has proven to beimpracticable, because the cellulose solution obtained in this way isunstable.

Despite these various approaches to obtaining a homogeneous and stablecellulose solution and to convey it through to the extrusion openings,while avoiding exothermic decomposition reactions, the environmentallyfriendly and economical production of a homogeneous cellulose solutionand its stability remain problematical. Furthermore, it is problematicalthat the cellulose solution ages, which is expressed in an increasingreduction in the degree of polymerisation with time. With somecelluloses which are supplied with already a low degree ofpolymerisation and are processed to form a spinning mass, ageing maylead to unacceptable reductions in quality.

The object of the invention is therefore to improve the known methodsand devices of the Lyocell technology such that, with the highestenvironmental compatibility, the method can be implemented independentof the type of cellulose used in a stable manner and with consistentlyconstant quality.

This object is solved for the aforementioned method in that the degreeof polymerisation of the cellulose, the cellulose suspension and/or thecellulose solution is monitored and that, in dependence of thedetermined degree of polymerisation, the residence period of thecellulose from its introduction to its extrusion is set.

For the aforementioned device, this object is solved according to theinvention by a monitoring device, through which a degree ofpolymerisation of the cellulose, the cellulose suspension and/or thecellulose solution can be monitored during the operation of the device,and by a control device, through which the processing duration from theintroduction of the cellulose to its extrusion in the spinning head canbe set in dependence of the measured degree of polymerisation.

The solution according to the invention is simple and enables any typeof cellulose to be used in the production of Lyocell fibres irrespectiveof its degree of polymerisation. Due to the control, according to theinvention, of the processing or residence duration of the cellulose fromits pulping through to its extrusion to give endless moulded bodies, auniform quality of the Lyocell fibres is obtained irrespective of thedegree of polymerisation of the processed cellulose. Since additionally,the degree of polymerisation is monitored according to the inventionduring the processing of the cellulose, it is no longer necessary toonly process carefully selected celluloses. According to the invention,any cellulose can now be processed, because a change of the degree ofpolymerisation in the cellulose, the cellulose suspension and/or thecellulose solution during the processing can be acquired and theprocessing duration adapted accordingly.

The method according to the invention and the device according to theinvention can both be used with the Lyocell method in which thecellulose is directly pulped in an aqueous solution of a tertiary amineoxide or in a tertiary amine oxide and cellulose solution is producedfrom it, and also with methods in which first a cellulose suspension isproduced containing essentially water and cellulose and only then is atertiary amine oxide or an aqueous solution of it added to form acellulose solution.

In an advantageous embodiment of the method according to the invention,the cellulose or the cellulose suspension is subjected to an enzymaticpretreatment. In the course of the enzymatic pretreatment a liquidenzyme preparation can, for example, be added in the ratio of 200:1referred to the cellulose content. As the liquid enzymatic preparation,a cellulase enzyme complex, for example Cellupract AL70 from the companyBiopract GmbH, or Cellusoft from the company Novo Nordisk, can be used.To prevent a too strong reduction in the degree of polymerisation, inparticular when celluloses with low initial degrees of polymerisationare processed to form Lyocell fibres, in an advantageous embodiment theduration of the enzymatic pretreatment can be set in dependence of thedegree of polymerisation of the introduced cellulose. In addition, thedegree of polymerisation can also be monitored during the enzymaticpretreatment and the duration of the enzymatic pretreatment can beshortened if the degree of polymerisation is strongly reduced. Theenzymatic pretreatment can be carried out in a pulper.

In order to monitor the polymerisation decomposition in an agitatingmachine, in particular during the production of the cellulose suspensionand/or during the enzymatic pretreatment, the known parameters (Newton'snumber, Reynold's number, Froude's number) in the agitating technologycan first be determined. Furthermore, the concentrations, temperatures,mixing times and the mixing quality can be accurately observed anddetermined in order to obtain information about the decompositionbehaviour during the formation of the emulsion or suspension. Thetemperature control can be carried out via temperature measurementdevices, e.g. of type Pt100. The exact concentration can be set usingcontinuous flow measurement devices for the emulsion or suspensionagent. The cellulose can be accurately measured using a differentialdosage weighing system and similarly continuously charged.

The suspension criteria, such as filling level, agitation duration andconcentration of the emulsion/suspension particles over the containerheight can be determined by the insertion of a measurement samplinglance into the suspension vessel or pulper or into the agitating machineand by the sampling of suspended material. It has been found to beparticularly advantageous if the form of the bottom of the suspensionvessel is realised as a dished or spherical bottom.

An impeller agitator is preferably used as the stirring element.However, propellers, inclined blade mixers, disc mixers, toothedstirrers, anchor stirrers as well as spiral stirrers or coaxialagitating machines can also be used. The shaft of the stirring machinewhich is connected to a drive motor is controlled and monitored forspeed. Similarly, the monitoring of the drive power, the input energyand the torque takes place during the emulsion and suspension processesfor the control of the enzymatic breakdown of the cellulose and thus forthe control of the degree of polymerisation using at least one sensor.

Surprisingly, it was found that during the addition of the enzyme,decomposition of the polymer chains took place and the rate ofdecomposition could be derived via the input emulsifying and suspensionpower or from power input by an agitating machine.

The degree of polymerisation of the cellulose, the cellulose suspensionand/or the cellulose solution can be determined by inline sensors suchas torque sensors with associated software and control with computationof the relative reduction in viscosity. These types of systems are, forexample, offered by the company PORPOISE. Alternatively, the degree ofpolymerisation can be determined by manual sample extraction followed byanalytical determination of the degree of polymerisation, which howeverreduces the degree of automation for the method.

As inline sensors, for example strain gauges, fitted between the drivemotor and agitating machine or eddy current sensors, can be used for themeasurement of power and torque. The eddy current measurement is basedon the principle that the permeability for magnetic field lines ismodified by mechanical stresses. The magnetic field generated by astationary sensor head penetrates the agitator drive shaft and induceselectrical voltages in the secondary coils of the sensor head inrelation to the mechanical stress which are proportional to the torque.The measurement takes place without contact and free of any reactiveforces.

A prerequisite for the use of this method in the suspension productionsystem is that a free, accessible piece of the shaft is available on thedrive side of the agitating machine to which the sensor can be aligned.

After a manual extraction of the sample, the degree of polymerisationcan be determined via the viscosity according to the cuoxam method. Themeasurements and the measurement signals from the inline sensors arecalibrated with the analytical results, correlated and are used for theprocess control and are to be preferred over a manual sample extraction,because they facilitate the automation of the control of the residenceduration.

During the processing of celluloses with a low degree of polymerisation,for example with a DP value of at the most 550, the enzymaticpretreatment is realised over a shorter time period than for a cellulosewith a higher degree of polymerisation, for example of at least DP 700.

The enzymatic pretreatment can last between 20 minutes and 80 minutes,whereby the degree of polymerisation after the termination of theenzymatic pretreatment does not drop below a DP value of 520. If the DPvalue falls below 520, the enzymatic pretreatment is terminated. Thenfollows the production of the cellulose solution through thevaporisation of water from the enzymatically broken down cellulose andthe aqueous tertiary amine oxide. Also in this process step further DPbreakdown can be monitored via inline sensors such as for example aninline viscosity device, e.g. from Porpoise, and can be controlled inconjunction with the enzymatic DP breakdown behaviour. In this respectthe rheometer is mounted in a branch of the pipe carrying the mass.Thus, a very fast online measurement with high precision is obtained.Through the measurement of the viscosity, the viscous flow, structuralviscosity, polymer swelling properties, elasticity and normal stressescan be derived, which in turn can be used for the control of theenzymatic breakdown.

In a further advantageous embodiment, the cellulose solution can betransported to the spinning head by a heated pipe system. Since areduction in the degree of polymerisation also occurs in the course ofthis transport, the transport speed of the cellulose solution in thepipe system can be set in relation to the degree of polymerisation ofthe cellulose solution, of the preceding cellulose suspension and/or theintroduced cellulose which is dissolved in the cellulose solution. Thelower the degree of polymerisation of the cellulose solution, the fasterthe cellulose solution is transported to the spinning head according tothe invention in order to prevent the extrusion of endless mouldedbodies with a degree of polymerisation which is too low. The duration ofprocessing the cellulose solution until its extrusion can be set in aparticularly easy way in an advantageous embodiment in that thetransport speed of a pump arrangement transporting the cellulosesuspension and/or cellulose solution is set in relation to the degree ofpolymerisation.

The processing duration of the cellulose from its introduction throughto its extrusion to form endless moulded bodies is preferably set suchthat it is at the most 80 minutes and/or at least 20 minutes. In thistime frame, a good solution of the cellulose in the tertiary amine oxidecan be achieved and at the same time the degree of polymerisation cannotfall below values which lead to an impaired quality of the Lyocellfibres.

In the following, an embodiment of the invention is described as anexample, with reference to the drawings. Here, features, as related toversions of the above individual advantageous embodiments of theinvention, can be arbitrarily combined with one another as required oralso omitted. In addition, the invention is documented based onexperimental examples.

It is shown in:

FIG. 1 an embodiment of a device according to the invention for theproduction of a cellulose solution in a schematic illustration, wherebythe method according to the invention can be implemented;

FIG. 2 a schematic illustration of the steps of the method for theproduction of the cellulose suspension;

FIG. 3 a schematic illustration of the change of the quantity of ironions borne out over time;

FIG. 4 a schematic illustration of the chemical oxygen demand in thepress water over time;

FIG. 5 a schematic illustration of a first method for the control of themetal ion content;

FIG. 6 a schematic illustration of the power transfer of an agitatingmachine for a cellulose suspension over the residence time;

FIG. 7 a schematic illustration of the reduction in the degree ofpolymerisation over the residence time for the cellulose suspension.

FIG. 1 shows a plant 1 for the production of endless moulded bodies 2,for example spinning filaments, from a spinnable cellulose solutioncontaining water, cellulose and tertiary amine oxide.

First, cellulose in the form of leaves or plates 3 and/or rolls 4 ispassed to a pulper 5 in batches. In the pulper 5, the cellulose 3, 4 istreated with water as a treatment medium, symbolically represented bythe arrow 6, and a cellulose suspension is formed, preferably stillwithout solvent or amine oxide. Enzymes or enzyme solutions can be addedfor the homogenisation and stabilisation of the cellulose suspension.

The quantity of the added water 6 is determined in relation to the watercontent of the cellulose. Typically the water content of the celluloseused is between 5 and 15 percent by mass. This variation is compensatedby changing the addition of water appropriately, so that the watercontent of the cellulose suspension or the bath ratio of solids/liquidremains approximately constant or attains a freely selected value.

From the pulper 5 the cellulose suspension is passed through a thickmatter pump 7 via a pipe system 8 to a press device 9, whereby thecellulose suspension of water and cellulose is preferably maintained ina temperature range from 60 to 100° C.

In the press device, the cellulose suspension produced by the pulper 5is for example expressed by rotating rolls 10. The expressed water orpress water 11 is collected by a collecting device 11′ and passed backto the pulper 5 by a conveying means 12, through an optional filterdevice 13 and through a mixing device 14 at least in part as water 6.The press device 9 can also be fitted with a suction device (not shown)for drawing off excess water from the cellulose suspension. In thisembodiment, the drawn-off water is passed back, as the press water, atleast in part to the pulper 5. For the purposes of this invention,drawn-off water or water removed from the cellulose suspension by othermeans is also press water which can be recycled for the treatment ordisintegration of the cellulose.

The filter 13 can comprise one or more surface filters, deep-bedfilters, membrane filters, plate filters, edge filters, separators,centrifuges, hydrocyclones, belt filters and vacuum belt filters, tubefilters, filter presses, rotating filters, reversible-flow filters,multilayer filters and also flotation methods. In addition, the presswater 11 can be osmotically treated in the filter 13; alternatively oradditionally metal ions and particles can be filtered out of the presswater 11 or metal-binding additives can be fed to the press water 11.

The respective proportions of the returned treatment medium 11 and offresh treatment medium 15, for example fresh water, fed from anotherfresh source are adjusted by a mixing device 14 in the water passed tothe pulper 5. In addition, the proportion of the treatment medium 11,which is passed or discharged out of the plant 1 through a waste waterpipe 16, is set by the mixing device 14.

The mixing device 14 can for example comprise a selector valve or anumber of valves. The mixing device 14 is controlled by a control device17 such that the proportions of the press water 11 and of the freshwater 15 in the water 6 fed to the pulper 5 can be set to variablyspecifiable values in response to an output signal from the controldevice via at least one control line 18.

After expressing, the cellulose suspension is transported furtherthrough the pipe system 8 to a stirring and conveying means 19 in whicha shear stress acting on the cellulose suspension is generated by astirring or conveying tool 20, such as screws, paddles or blades. Forthe stirring and conveying means 19, no annular layer mixers can beemployed, such as originating from DRAIS Misch- und Reaktionssysteme andsold under the designation CoriMix®. The annular layer mixers are onlyused for moistening or impregnating dry cellulose materials which arenot used in the method described here.

In the region of the shear stresses of the stirring and conveying means19, in the so-called shear zone, a treatment medium such as tertiaryamine oxide, in particular N-methylmorpholine-N-oxide, is supplied tothe cellulose suspension in aqueous form with a molar ratio NMMNO/H₂O ofbetween 1:1 and 1:2.5 as solvent for the cellulose via a pipe 21. Inaddition, additives such as stabilisers and enzymes, organic additives,delustering substances, alkalis, solid or liquid earthy bases, zeolites,finely pulverised metals such as zinc, silver, gold and platinum for theproduction of anti-microbial and/or electrically or thermally conductingfibres during and after the spinning process and/or dyes can be added tothe cellulose suspension in the shear zone. The concentration of theadditives can be controlled in the range from 100 to 100,000 ppmreferred to the fibre product.

The concentration of the supplied NMMNO depends on the water content ofthe celluloses 3, 4 currently in the cellulose suspension. The stirringand conveying means 19 acts as a mixer in that the tertiary amine oxideis mixed with the cellulose suspension and the cellulose solution isproduced. Then the cellulose solution to which NMMNO has been added istransported via the pipe system 8 to a second stirring and conveyingmeans 22. The stirring and conveying means 22 can comprise avaporisation stage. From the stirring and conveying means 22 on, thepipe system can be heated. In contrast to the unheated pipe system 8,the heated pipe system in FIG. 1 is given the reference symbol 8′. Inparticular, a pipe system can be used, as described in WO 01/88232 A1,WO 01/88419 A1 and WO 03/69200 A1. Conveying devices 7′ in the form ofpumps can be arranged both in the pipe system 8 as well as in the pipesystem 8′ to transport the cellulose suspension and/or the cellulosesolution.

After the addition of the tertiary amine oxide, the metal ion content ofthe cellulose solution, in particular copper and iron ions, in the pipe8′ and/or in at least one of the shear zones 19, 22, or before and/orafter one of the shear zones is measured using the sensors 23, and asignal representing the metal content or the content of individualdestabilising metal ions, such as iron, chrome, copper and/or molybdenumis output to the control device 17. Alternatively or in addition to anautomatic inline sample extraction, the metal ion content can in afurther embodiment be determined after a manual sample extraction usingwet-chemical methods in an automatic laboratory analysis device andpassed on from there to the control device 17 automatically or manually.However, with a manual sample extraction compared to the automaticinline sample extraction directly from the pipe systems 8, 8′, there isthe disadvantage that the feedback to the controller for the metal ioncontent contains a manual process stage and cannot therefore beautomated.

The control device 17 compares the metal ion content measured by thesensors 23 with predetermined limits and outputs a signal depending onthis metal ion content to the mixing device 14. Due to the controlsignal to the mixing device 14, the composition of the water 6 astreatment medium passed to the pulper 5 is set in dependence of thecontent of the destabilising metal ions of the cellulose solution, andthe metal content or the content of individual metal ions in thecellulose solution to which tertiary amine oxide has been added isregulated under closed-loop control to a predetermined value. Since theconcentration of reactions in the cellulose solution increases after thevaporisation stage, preferably a sensor is provided which monitors themetal content of the cellulose solution after the addition of allconstituents and after all the vaporisation stages.

If, for example, the metal content of destabilising metal ions in thecellulose solution, as acquired by the sensors 23 or by usingwet-chemical methods, is too high, then the proportion of fresh water inthe water 6 fed to the pulper 5 is increased. The metal content is thenadjusted by the control device 17 such that it remains below a stabilitylimit of 10 mg/kg. The metal content can also be determined before theformation of the cellulose solution, i.e. still in the cellulosesuspension, whereby this measurement is more appropriate than themeasurement of the metal content directly in the cellulose solution.

As sensors 23 devices for atomic absorption measurement, massspectrometers, optical detectors for the acquisition of fluorescencespectra, emission spectra or Raman scattering can be used. These typesof sensor are known and are produced by various manufacturers, forexample by Perkin Elmer.

Furthermore, the degree of polymerisation of the cellulose suspensionand/or the cellulose solution can be determined via inline sensors 23′,for example from the company PORPOISE, the number of which is asrequired, for example via a viscosity measurement. Instead of the inlinesensors 23′, a sample can be taken manually at the corresponding pointand its degree of polymerisation be found in the normal way, for exampleaccording to the cuoxam method. Furthermore, as the sensors 23′, sensorscan be used which monitor the power transferred by agitating machines,such as torque sensors, eddy current sensors or strain-gauge sensors.

As shown in FIG. 1, the sensors 23′ for the degree of polymerisation arearranged in the pulper 5 and in the pipe system 8, 8′. The measureddegree of polymerisation is passed in signal form to the control device17. The inline sensors are rheometers from the company PORPOISE.

During the control of the composition of the water 6, the control device17 takes into account the previously determined metal content of thecellulose 3, 4 passed to the pulper 5. For this, the analysed metalcontent of individual metal ions or the complete content of metal in thecellulose 3, 4 just used can be entered into the control device 17and/or its degree of polymerisation via an input device 24. Thepreadjustment or present default value of the metal content is takeninto account in the determination of the proportions of the press waterand fresh water in the water fed to the pulper 5. For example, withcellulose having a high metal content a higher proportion of fresh water15 is passed to the pulper 5 at the start or certain metal-bindingadditives are mixed into the cellulose suspension.

If the metal content decreases, as it is acquired by the sensors 23 inthe cellulose solution to which tertiary amine oxide has been added,below a certain limit which is taken as sufficient for protectionagainst exothermic reactions, for example 10 mg/kg, then the proportionof press water in the water passed to the pulper 5 is increased.Consequently, with sufficient protection against exothermic reactions,less fresh water is consumed and less press water is discharged to theenvironment.

The control device 17 also controls the residence or processing periodof the cellulose 3, 4 in the plant 1 in dependence of the degree ofpolymerisation as entered manually into the input device 24 or, duringoperation, determined by the sensors 23′ or by means of manuallyextracted samples analysed in the laboratory. The sensors 23′ and/or thelaboratory analysis devices for the samples extracted manually act asmonitoring devices for the degree of polymerisation. The processingduration of the cellulose from its introduction into the pulper 5through to its extrusion in an extrusion head 25 is set such that closeto the extrusion head 25, shortly before the extrusion of the cellulosesolution, the degree of polymerisation does not fall below 450 DP,preferably not below 550 DP. If a cellulose 3, 4 is processed, whichalready has a low degree of polymerisation, then the transport speed ofthe cellulose suspension and the cellulose solution in the pipe system8, 8′ is increased, whereby the residence period of the cellulose inplant 1 is reduced.

The control device 17 controls in particular the conveying device 7which empties the pulper 5. With celluloses having a low degree ofpolymerisation, the pretreatment and prepulping are shortened byoperating the pump 7 at an earlier time. Simultaneously, the conveyingcapacity of the further pumps 7′ in the pipe systems 8, 8′ is increased.The duration of the pretreatment in the plant 1 takes, for example withcelluloses 3, 4 having a high degree of polymerisation of at least 600DP, about 40 minutes, and, with celluloses 3, 4 having a relatively lowdegree of polymerisation of 400 DP and less, at the most 25-30 minutes.

After the agitation and conveying means 22, the now extrudable cellulosesolution is passed to the extrusion head 25, which is fitted with alarge number of extrusion openings (not shown). The highly viscouscellulose solution is extruded through each of these extrusion openingsto form an endless moulded body 2 into an air gap 26. An orientation ofthe cellulose molecules occurs due to a drawing of the cellulosesolution which is still viscous after the extrusion. To achieve this,the extruded cellulose solution is drawn off the extrusion openings by atake-off mechanism 27 with a speed which is greater than the extrusionspeed.

After the air gap 26 the endless moulded bodies 2 cross a precipitationbath 28 containing a non-solvent such as water, whereby the cellulose inthe endless moulded bodies 2 is precipitated. In the air gap 26, theendless moulded bodies 2 are cooled by a cooling gas flow 29. Here, incontrast to the teachings of WO 93/19230 A1 and EP 584 318 B1, it hasbeen found substantially more advantageous if the cooling gas flow doesnot impinge on the endless moulded bodies 2 immediately after the exitof the endless moulded bodies 2 from the die, but rather at a distancefrom the die. In order to achieve the best fibre properties, the coolinggas flow should be turbulent and exhibit a velocity component in theextrusion direction, as described in WO 03/57951 A1 and in WO 03/57952A1.

The precipitation bath 28 becomes increasingly enriched with tertiaryamine oxide, so that it must be continuously regenerated using arecovery device 30. For this, in operation, the liquid from theprecipitation bath is fed to the recovery device 30 via a pipe 31, whichfor example is connected to an overflow on the precipitation bath. Therecovery device 30 removes the tertiary amine oxide from the liquid andreturns purified water via a pipe 32. Waste substances that cannot berecycled are ejected from the device 1 via a pipe 33 and taken fordisposal.

In the recovery device 30, the amine oxide is separated from the waterand passed via a pipe 34 to a further mixing device 35, to which freshamine oxide is fed via a pipe 36. The regenerated amine oxide from thepipe 34 is mixed with the fresh amine oxide 36 and passed via the pipe21 to the shear zone 19.

Metal ions can be removed from the regenerated amine oxide by an ionexchanger, for example from the company Rohm und Haas, Amberlite GT 73or filters 37.

The mixing device 35 and the purification device 37 can be controlled bythe control device 17 in dependence of the metal ion content as measuredby the sensors 23.

Then, the endless moulded bodies are treated further, for examplewashed, brightened, chemically treated in a device 38 to influence thecross-linking properties, and/or dried and pressed out further in adevice 39. The endless moulded bodies can also be processed by a cuttingdevice, which is not shown, to form staple fibres and passed innon-woven form from the device 1.

All of the conveyance of the cellulose solution in the pipe system 8′occurs continuously, whereby buffer containers 40 can be provided in thepipe system 8′ to take up variations in the conveyed amount and/or ofthe conveying pressure and to facilitate continuous processing withoutthe occurrence of dead water regions. The pipe system 8′ is equippedwith a heating system (not shown) to maintain the cellulose solutionduring conveyance at a temperature at which the viscosity issufficiently low for economical transport without decomposition of thetertiary amine oxide. The temperature of the cellulose solution in thepipe section 8′ is between 75 and 110° C.

At the same time, the high temperature promotes the homogenisation anduniform mixing, which can be increased by static or rotating mixers.

The residence time of the cellulose suspension or solution in the pipesystem 8, 8′ from the thick matter pump 7 through to the extrusion head25 can be, depending on the degree of polymerisation of the processedcellulose and with the use of special additives for the cellulosesuspension and cellulose solution, between 5 minutes and 2 hours,preferably about 30 to 60 minutes. The implementation of the methodaccording to the invention is now described based on experimentalexamples.

In order that the required enzymatic breakdown or decomposition of thecellulose can be reliably set also in larger plants, the production ofthe suspension was examined more closely in laboratory experiments,because the mixing and stirring stages occur in a very complex mannerand turbulence mechanisms can also affect the rheology of the treatedproduct. Therefore, before transferring to large scale, it was necessaryto specifically investigate the enzymatically controlled breakdownbehaviour (reduction of DP).

In the laboratory model experiments, the parameters (Newton's number,Reynold's number, Froude's number) known in agitation technology weredetermined. The concentrations, temperatures, mixing times and mixingquality were exactly observed and determined in order to obtaininformation about the breakdown or decomposition behaviour during theformation of the emulsion or suspension. The exact adjustment of theconcentration occurred through continuous flow measurement devices foremulsion or suspension agents, and the addition of the cellulose alsooccurred continuously, exactly measured via a differential dosageweighing system. The suspension criteria, such as filling level,stirring duration and concentration of the emulsion/suspension particlesover the container height were determined by inserting a measurementsampling lance and the extraction of suspended material. An impellerstirrer was used as the stirring element. However, propellers, inclinedblade mixers, disc mixers, toothed stirrers, anchor stirrers as well asspiral stirrers or coaxial agitating machines can also be used. Theshaft of the stirring machine which is connected to a drive motor iscontrolled and monitored for speed. Similarly, the monitoring of thedrive power, the input energy and the torque occurs during the emulsionand suspension stages for the control of the enzymatic breakdown of thecellulose and thus for the control of the degree of polymerisation.

A first series of experiments involved the cellulose pretreatment forthe production of the cellulose suspension and the examination of thepress water. In the following reference is made to the schematicillustration of the pretreatment in FIG. 2 and also the referencesymbols of FIG. 1 are used.

EXPERIMENTAL EXAMPLE 1

In a process step A cellulose 3, 4 (cf. FIG. 1) of type MoDo DissolvingWood Pulp, pine sulphite wood pulp, was placed in a pulper 5 from thecompany Grubbens having a net filling volume of 2 m³ with water 6 in amixing ratio of 1:17 (solid density 5.5%). The cellulose exhibited acuoxam DP of 650 and an α-cellulose content greater or smaller than 95%.Commercially available celluloses based on hardwood or softwood can beused. Hemicellulose contents in the cellulose in the range from 2 to 20%can also be processed in the method. Other possible celluloses are SappiEucalyptus, Bacell Eucalyptus, Tembec Temfilm HW, Alicell VLV andWeyerhäuser α-cellulose of less than 95%. The fed water 6 consisted of30 parts of fully desalinated fresh water 15 and 70 parts of presswater.

Under vigorous stirring, technically pure formic acid 50 in the ratio of1:140 and a liquid enzyme preparation 51 in the ratio of 1:200, referredin each case to the cellulose content, were added. An enzymaticpretreatment was then carried out for a duration of about 35 minutesuntil a homogeneous cellulose suspension was obtained. A cellulaseenzyme complex, such as for example Cellupract® AL 70 from Biopract GmbHor Cellusoft from Novo Nordisk can be used as the enzyme preparation 52.

Then, the pretreatment was interrupted in a process step B by theaddition of sodium hydroxide solution (soda lye) 52 in the ratio of1:500 referred to the cellulose content of the cellulose suspension inthe pulper 5.

The cellulose suspension was then dehydratised to about 50% in a processstep C in a vacuum belt filter acting as press means 9 followed by anexpressing system from the company Pannevis, so that the expressedcellulose exhibited a dry content of about 50%. From step C, theexpressed cellulose was then passed on via the pipe 8 for production ofthe cellulose solution containing NMMNO, water and cellulose. Thesesteps are not shown in FIG. 2 for the sake of clarity.

The press water was collected in the press means 9 and led away via thepipe 11 (cf. FIG. 1). Approximately 75% of the press water was fed backto the pulper 5 and about 25% of the press water was passed via the pipe16 to a waste water purifier.

The degree of polymerisation of the cellulose was always selected suchthat a DP (degree of polymerisation) of about 450 to about 550 wasobtained in the spinning solution. The cellulose concentration was setto about 12% in the spinning solution.

The press water remaining in the system 1 was again mixed in a mixingdevice 14 (cf. FIG. 1) in a process step D with the fully desalinatedwater, as described above.

EXPERIMENTAL EXAMPLE 2

In another experiment, all the steps of Experimental Example 1 wererepeated, except that in process step A, the quantity of the addedenzyme preparation was reduced to 1:125 referred to the cellulosecontent of the cellulose suspension.

EXPERIMENTAL EXAMPLE 3

In another experiment, the steps of Experimental Examples 1 and 2 wererepeated, except that in process step A no enzyme preparation was added.

Results of the Experimental Examples 1 to 3

To check the effectiveness of the method according to the invention, thepress water collected during the expressing stage was analysed forcopper and iron ion content and additionally the chemical oxygen demandwas determined.

As a result of this experiment, it can be summarized that, in the firstpulp cycles, the measured values of the ingredients increase due to thecirculation of a part of the press water. Since, however, a part of thepress water is permanently transferred out together with the ingredientsdissolved therein, a steady state sets in after some time in which theamount of ingredients or content substances, in particular the metalions, remains constant.

In total about 10% of the iron ions introduced by the cellulose 3, 4 andabout 40% of the copper ions introduced by the cellulose was removed bythe press water feedback. In a continuous plant operation with a returnof the press water, the percentage proportion of the iron extracted fromthe system 1 may be between 22% and 35% referred to the quantity of ironintroduced by the cellulose.

FIG. 3 gives a schematic temporal trace of the iron ion extraction.

The stable final state of the system 1 is achieved, as ExperimentalExamples 1 to 3 show, independent of the amount of introduced enzymesfor the pretreatment of the cellulose.

This is also confirmed by the temporal change of the chemical oxygendemand (COD), as illustrated in FIG. 4. The chemical oxygen demand wasdetermined in the press water according to DIN 38409 and approximateswith increasing duration of the press water feedback to a constantvalue.

Furthermore, the degree of polymerisation and the DP reduction as wellas the onset temperature of the spinning solution were determined asindicators of stability in the cellulose solutions obtained according toExperimental Examples 1 to 3. The results of the experimental examplesare shown in Table 1.

TABLE 1 Experimental DP reduction T_(onset) Example [%] ° C. 1 27.5 1652 27 165 3 9 160

As shown in Table 1, the cellulose solution obtained through press waterfeedback is stable and exhibits an onset temperature of at least 160° C.The onset temperature shown in Table 1 according to the method withpress water return according to the invention is also above the onsettemperature as it is obtained by the method of WO 95/08010, and inpractice is about 150° C.

Based on these investigations, it can be seen that despite the presswater feedback, the onset temperatures still lie above the onsettemperatures for the dry processing of cellulose and can be increased byan enzymatic pretreatment of the cellulose. This means that the presswater feedback is suitable for industrial use.

In another series of experiments the effect of the substances containedin the press water on the stability of the cellulose solution wasinvestigated. To achieve this, a concentrate of 5 l of press water inthe ratio 1:270 was added to the cellulose solution in each of theExperimental Examples 1 and 3 and feedback of the press water wasomitted.

In both cases, once according to the method of Experimental Example 1without enzymatic pretreatment and once according to the method ofExperimental Example 3 with enzymatic pretreatment, a reduction of theonset temperature to about 141° C. occurred in each case due to thepress water concentrate. Thus, it is demonstrated that the press waterfundamentally reduces the stability of the cellulose solution.

This destabilisation of the cellulose solution can however be preventedby the outward transfer of the treatment medium with the destabilisingmetal ions. The proportion of the returned treatment medium depends onthe type of cellulose used, as shown in the following table.

The iron and copper content as well as the metal ion content of thecellulose overall varied noticeably with the various types of cellulose,as can be seen from Table 2. The metal content of the various types ofcellulose was determined by incineration in the platinum crucibleaccording to DIN EN ISO 11885 (E22) and with flame AAS.

TABLE 2 Used cellulose Contained substances Cellulose 1 Cellulose 2Cellulose 3 Cellulose 4 Cellulose 5 Cellulose 6 Cellulose 7 Cellulose 8in cellulose mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg Fe 1.3 2.01.6 5.8 2.2 2.6 14 13 Mn <0.3 <0.1 0.2 0.33 n.d. <0.3 0.4 <0.3 Mg 2 2226 32 138 2 21 7.8 Co 0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 Ca 54 4 3764 30 6 130 27 Cr <0.3 <0.3 1.4 <0.3 <0.3 0.4 <0.3 <0.3 Mo <0.3 <0.1<0.1 <0.1 <0.3 <0.3 <0.3 <0.3 Ni <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3Cu 0.3 <0.2 0.2 <0.3 <0.3 <0.3 0.3 0.3 Na 396 48 93 92 263 176 335 8.2

In a final series of experiments the schematic experimental set-up ofFIG. 5 was used. In FIG. 5 the reference symbols of FIGS. 1 and 2 areused for elements with similar or the same function.

With the set-up in FIG. 5 the amount of press water returned to thepulper 5 was adjusted to the iron and copper content of the expressedcellulose.

With the arrangement of FIG. 5, the iron ion and copper ion content wasmeasured as representative values for the metal ion content by thesensors 23, 23′ (cf. FIG. 1).

Due to the control of the proportion of the press water in the water 6fed to the pulper 5, the iron concentration was maintained as closely aspossible below 10 mg/kg absolutely dry and the copper concentration justbelow 0.5 mg/kg absolutely dry. These values were possible for anadequate stability of the cellulose solution in the pipe 8 withsimultaneous maximum retention of the press water within the system 1and consequently with minimum outward transfer of the press water 16from the system 1.

The control of the metal ion content occurred in such a way that if oneof these two limits was exceeded, the amount of press water outwardlytransferred from the system 1 and passed on for waste water purificationwas increased by opening a valve 58. At the same time, closure of thevalve 59 reduced the proportion of press water fed back in thepretreatment stage.

If a direct pulping of the cellulose 3, 4 occurs in amine oxide, thenthe setting of the metal ion content according to the invention can alsobe achieved via the tertiary amine oxide recovered from the spinningbath 28. In this respect the degree of purification on the metal ionfilter 37 and/or the proportion of the tertiary amine oxide 36 freshlyadded to the regenerated tertiary amine oxide 34 can be set independence of the metal ion content as measured by the sensors 23 and23′, as well as in dependence of the metal ion content previously foundin the cellulose 3, 4. The control functions similarly as with the presswater feedback.

In a modification of the method described in FIG. 1, in the recoverydevice 30 recovered water from the spinning bath 28 can be returnedinstead of or together with the press water to the pulper 5.

The metal ion filter 37, as it is used in the recovery of the tertiaryamine oxide from the spinning bath 28, can of course also be used forthe purification of the returned press water.

EXPERIMENTAL EXAMPLE 4

In this experimental example, the method was carried out according toExperimental Example 1 and the DP value in the cellulose was measured atvarious points in the enzymatic treatment, in the cellulose suspensionand in the cellulose solution. Table 3 shows the figures obtained. Fromthis table it can be seen that through to the extrusion of the endlessmoulded body the degree of polymerisation is reduced by 20% fromoriginally 628 in the cellulose 3, 4 to 504. A substantial proportion ofthe reduction in the degree of polymerisation is provided by theenzymatic pretreatment, which in addition offers the advantage that itsduration correlates well with the polymerisation breakdown. Thereduction in the degree of polymerisation can be prevented in a simplemanner if the duration of the enzymatic treatment is set such that forexample a degree of polymerisation of at least 590 DP to 600 DP ispresent at the end of the enzymatic treatment. Furthermore, thetransport of the cellulose suspension and/or the cellulose solution canbe speeded up such that a degree of polymerisation of at least 550 DP isobtained before the extrusion of the cellulose solution.

TABLE 3 DP value Overall reduction Cellulose 628   0% Enzymatictreatment 30 minutes 603 4.14% Enzymatic treatment 45 minutes 592 5.73%Enzymatic treatment 60 minutes 582 7.32% Cellulose suspension 582 7.32%Cellulose solution 538 14.33%  Endless moulded bodies 504 19.75% 

EXPERIMENTAL EXAMPLE 5

In this experiment the method was implemented similar to ExperimentalExample 4, but only the activation step was observed and the DP value inthe cellulose suspension, in the spinning solution and in the endlessmoulded body was not measured.

The starting cellulose was selected with a DP of 780 and the enzymeconcentration was increased by the factor 0.5. Furthermore, the powerconsumption of the agitating machine was also recorded during theactivation step and scaled to the container volume and the solidsconcentration. Table 4 shows the figures measured when carrying out theexperiment.

TABLE 4 Power consumption Treatment [kW/m³ of input Cellulose Powertransferred [kW/kg DP of Step in the method time [s] liquid] content [%]of cell.] cell. Input liquid 300 0.4 0 — Cellulose feed 1 360 0.49 1.0%0.0049 780 Cellulose feed 2 420 0.52 2.0% 0.0104 780 Cellulose feed 3480 0.8 3.0% 0.0240 780 Cellulose feed 4 540 1.81 4.0% 0.0724 780Cellulose feed 5 600 2.55 5.0% 0.1275 780 Homogenisation 660 2.53 5.0%0.1265 780 Homogenisation 720 2.4 5.0% 0.1200 780 Homogenisation 780 2.55.0% 0.1250 780 Enzyme addition 780 2.51 5.0% 0.1255 780 Homogenisation840 2.53 5.0% 0.1265 780 Homogenisation 900 2.5 5.0% 0.1250 773Homogenisation 960 2.41 5.0% 0.1205 765 Homogenisation 1020 2.3 5.0%0.1150 751 Homogenisation 1080 2.17 5.0% 0.1085 747 Homogenisation 13801.88 5.0% 0.0940 701 Homogenisation 1680 1.65 5.0% 0.0825 657Homogenisation 1980 1.52 5.0% 0.0760 613 Homogenisation 2280 1.5 5.0%0.0750 569 Homogenisation 2580 1.45 5.0% 0.0725 522

FIG. 6 illustrates the trace of the power in kilowatts per kilogram ofcellulose applied by the agitating machine over the treatment period ofthe cellulose suspension in the agitating machine, FIG. 7 shows thechange of the degree of polymerisation in the agitating machine.

As can be seen from Table 4 and FIGS. 6 and 7, the degree ofpolymerisation reduces with increasing treatment time for the cellulosesuspension.

The reduction in the degree of polymerisation can be seen from theclearly reduced power applied by the agitating machine. With a reduceddegree of polymerisation the power applied in the agitating machine alsoreduces.

Consequently, during the production of the cellulose suspension, inparticular during the enzymatic pretreatment, in the agitating machinethe power of the agitating machine can be used as a representativequantity for the degree of polymerisation in the control of the degreeof polymerisation. The same method for monitoring the degree ofpolymerisation is also possible with the following agitating machines.If, for example, the power of the agitating machine falls under acertain predetermined value, e.g. a limit determined by experiments,this is a sign that the degree of polymerisation is falling or hasfallen below a figure specified for this processing stage. As aconsequence, the remaining residence time of the cellulose suspensionand/or the cellulose solution in the process step is shortened.

1. A method for producing Lyocell fibers comprising a cellulose with apredetermined degree of polymerization is introduced producing acellulose solution, or initially a cellulose suspension and from thisthen the cellulose solution, from the cellulose with the addition of atreatment medium; extruding the cellulose solution to form endlessmolded bodies; monitoring the degree of polymerization of the cellulose,the cellulose suspension, the cellulose solution or combination thereof;and setting the residence time of the cellulose from its introductionthrough to its extrusion dependence of the measured degree ofpolymerization.
 2. The method according to claim 1, wherein thecellulose suspension is subjected to an enzymatic pretreatment.
 3. Themethod according to claim 2, wherein the duration of the enzymaticpretreatment is adjusted in dependence of the degree of polymerizationof the introduced cellulose.
 4. The method according to claim 3, whereinthe enzymatic pretreatment for a cellulose with a lower degree ofpolymerization is carried out over a shorter time than for a cellulosewith a higher degree of polymerization.
 5. The method according to claim2, wherein the enzymatic pretreatment is set between 20 minutes and 80minutes.
 6. The method according to claim 2, wherein for the enzymaticpretreatment at least one cellulase enzyme complex is added to thecellulose suspension.
 7. The method according to claim 1, wherein thecellulose solution is conveyed through a heated pipe system forextrusion and that the conveying speed of the cellulose solution in thepipe system is set in dependence of the degree of polymerization.
 8. Themethod according to claim 7, wherein during the processing of acellulose with a lower degree of polymerization in the cellulosesolution the conveying speed of the cellulose solution is set higherthan the conveying speed during the processing of a cellulose in thecellulose solution with a higher degree of polymerization.
 9. The methodaccording to claim 1, wherein the residence duration of the cellulosefrom its introduction through to its extrusion in the cellulose solutionis at most two hours.
 10. The method according to claim 1, wherein theresidence time of the cellulose from its introduction through to itsextrusion in the cellulose solution is at least five minutes.
 11. Themethod according to claim 1, wherein the conveying speed of the pumparrangement conveying the cellulose suspension, the cellulose solutionor combination thereof is set in dependence of the degree ofpolymerization in the cellulose, the cellulose suspension, the cellulosesolution or combination thereof.
 12. The method according to claim 1,wherein the degree of polymerization of the cellulose solution shortlybefore the extrusion is set to a DP value of at least 450 DP to 550 DP.13. The method according to claim 1, wherein the duration of anenzymatic pretreatment of the cellulose is set such that its DP value isat least 500 immediately after termination of the enzymaticpretreatment.
 14. The method according to claim 1, wherein the power ofan agitating machine is monitored as a representative quantity for thedegree of polymerization of the cellulose suspension.
 15. A device forthe production of Lyocell fibers, comprising a mixing device, to which acellulose can be fed and in which a cellulose solution directly or withthe formation of a cellulose suspension can be processed with theaddition of a treatment medium, a spinning head through which thecellulose solution can be extruded to form endless molded bodies, aconveying device through which the cellulose suspension cellulosesolution or combination thereof can be conveyed from the mixing deviceto the spinning head, a monitoring device, through which a degree ofpolymerization of the cellulose, of the cellulose suspension, of thecellulose solution or combination thereof can be monitored during theoperation of the device, and a control device, through which theprocessing duration from the introduction of the cellulose through tothe extrusion in the spinning head can be set in dependence of themeasured degree of polymerization.
 16. The device according to claim 15,wherein the conveying capacity of the conveying device is designed to becontrolled by the control unit in dependence of the degree ofpolymerization.
 17. The device according to claim 15, wherein a sensoris provided through which the power of an agitating machine can bemonitored.